yeah its well easy. You just need some kind of transponder like those PC locks that unlock your PC when you are near but i would go with a standalone device. Don't that phigit place do rf tags things. you want something like that but longer range.
One concern i would have though is that car makers probably go to great lengths to make sure they use an uncommon frequency range to prevent other devices opening your car and lots of code encryption. Also i would be concerned that if the battery went flat in the device in your pocket the engine cuts out at 100 on the motorway and you ar eleft with no power steering and no brake assist. You would have to do it so the engine doesnt need the imobiliser when running but it would be cool if you could have a sensor on the gear **** to check it is in neutral and on the hand brake to enable remote start form the keypad too for those quick getaway from one of your girlfriends when she finds out you have others!